The present invention relates to turbomachinery seals of the type used in gas turbine engines and rocket turbopumps and, more particularly, to turbomachinery brush seals and plate seals.
Significant challenges exist in sealing between static and high speed rotating parts in turbomachinery so as to limit fluid flow from a high pressure region to a low pressure region along the rotating part. Fluid leakage between high pressure and low pressure regions is undesirable because it wastes fluid (e.g., air, combustion gas, steam, etc.), causing a loss in power and efficiency. Yet with pressure differentials between high pressure and low pressure regions in known turbomachinery exceeding 1,000 psi and with rotational speeds in excess of 30,000 rpm for gas generator rotors in small engines and in rocket turbopumps, designing effective and long-lasting seals for turbomachinery has been problematic.
These design challenges are complicated by the fact that during transient operating conditions, e.g., during aircraft takeoffs and landings, various engine parts experience different thermal growth rates. As a result, excessive rubbing between seals and rotating parts can occur giving rise to excess wear of these components. Thus, a seal that provides adequate longevity and sealing capability during steady state operation, may not function satisfactorily under transient operating conditions.
Several types of seals have been developed to restrict the flow of fluid from a high pressure region to a low pressure region in turbomachinery. One type, labyrinth seals, are positioned between rotating and stationary members at the interface of high and low pressure regions. A labyrinth seal typically consists of one or more hardened seal teeth disposed on a rotating, substantially cylindrical, member and running close in proximity to a cylindrical or stepped cylindrical stator or stationary member. The seal teeth act to restrict the flow of fluid between the high pressure and low pressure regions.
Labyrinth seals generally perform well from the perspective of pressure differentials they are capable of sealing. However, labyrinth seals are deficient with respect to the relatively high amount of leakage that occurs through the seal. Labyrinth seals have leakage rates that are 50% to 90% greater than contemporary brush seals.
A second type of seal used in turbomachinery is brush seals. These seals consist of a plurality of compliant bristles which extend generally radially inwardly from an annular ring to which they are attached. The ring is typically clamped or otherwise secured to a static member and circumscribes and is mostly concentric with a rotating member. The brush seal is sized so that the bristles are biased against the shaft, typically being angled in the direction of rotation of the shaft. During operation the bristles rub against the shaft, compliantly deforming due to centrifugal and thermal growth and eccentric rotation of the shaft, thereby maintaining their sealing capability. As noted above, in contrast to labyrinth seals, brush seals typically have significantly lower leakage rates.
Various improved brush seal designs have been developed with a goal of sealing ever-increasing pressure differences between adjacent high pressure and low pressure regions. Atkinson et al. disclose in U.S. Pat. No. 5,106,104, a multiple stage brush seal consisting of a plurality of seal assemblies that include a front plate, a plurality of bristles and a backing plate which has a greater radial length than the front plate. Each seal assembly is axially spaced from adjacent seal assemblies. The bristle density decreases from the high pressure side to the low pressure side of the seal, while the spacing between the backing plates and the rotor increases from the high pressure side to the low pressure side of the seal.
Kelch et al. disclose in U.S. Pat. No. 5,201,530 a unitary brush seal having multiple layers of bristles. The packing density of the bristles decreases from the high pressure side to low pressure side of the seal, while the clearance between the bristles and the rotor increases from the high pressure side to the low pressure side of the seal.
While brush seal designs in these patents may represent an advance in the art, they suffer from a significant short coming known to all brush seals. Specifically, it is believed all known brush seals are incapable of restricting the flow of fluid from a high pressure region to a low pressure region in turbomachinery where (a) the pressure differential exceeds about 250-300 psi and (b) the flow through or xe2x80x9cleakagexe2x80x9d past the seal is less than 10% of what would occur in the annular space where the brush seal is exposed, i.e., are axially unsupported by backing plates or other structure, if the seal was not present in this annular space. This limitation is significant as advances in turbomachinery, i.e., particularly gas turbine engines and turbo rocketpumps, require sealing of larger pressure differentials between high pressure and low pressure regions as a consequence of increased performance demands.
Hendricks et al. in the article xe2x80x9cBidirectional Brush Seals-Post Test Analysis,xe2x80x9d NASA Technical Memorandum 107501, November, 1997, disclose in illustrations on page 11 two bidirectional brush seal designs. The brush seals are bidirectional in the sense that the high pressure region may confront either side of the brush seal. The Hendricks et al. brush seal designs feature a bristle bed and a pair of annular backing plates on opposite sides of the bristle bed. The backing plates have an equal radial length which is less than the radial length of the bristle bed. In the outer brush seal version of the Hendricks et al. brush seal, the axial thickness of the bristle bed increases from the radially innermost end to the radially outermost end. The inner brush seal version of the Hendricks et al. brush seal tapers oppositely, i.e., its axial thickness decreases from its radially innermost end to its radially outermost end. In both cases, only a relatively small portion, i.e., about 10%, of the entire length of the bristle bed is in direct contact with the annular backing plates. The majority of the radial length of the bristle bed, i.e., about 80% of its radial length, is surrounded by the annular plates, but is spaced from the plates by an axial distance equal to about the axial thickness of the bristle bed at its axially thinnest point. Only a relatively small portion, i.e., about 10%, of the radial length of the bristle bed is not supported in or surrounded by the annular backing plates.
While the Hendricks et al. brush seals are bidirectional, it is believed they continue to suffer from some of the problems discussed above with respect to known brush seals. In particular, it is believed the axial gap between the bristle bed and adjacent backing plates which extends over the majority of the radial length of the bristle bed will cause bending and bunching up of the discrete bristles. This bunching up can create gaps in the unsupported portion of the bristle bed through which fluid leakage can occur.
As a result of these limitations in known labyrinth seals and brush seals, improvements in turbomachinery, particularly rocket turbopumps, has been impeded.
One aspect of the present invention is a seal assembly designed for use with a first member, e.g, a shaft or impeller hub. The assembly comprises a support with a central axis and a seal attached to the support so that a portion of the seal extends radially beyond the support. This portion of the seal defines an annular space. The seal is sized to engage the first member so as to permit relative rotational movement between the first member and the seal. The seal, when positioned between a high pressure region and a low pressure region and engaging the first member, maintains a pressure difference of greater than 300 psi between the high pressure region and the low pressure region while permitting relative rotational movement between the first member and the seal and limiting leakage between the high pressure region and the low pressure region such that fluid flow from the high pressure region to the low pressure region through the seal does not exceed thirty percent of what such fluid flow would be through the annular space if such portion of the seal did not occupy the annular space.
Another aspect of the present invention is a plate seal designed for use with a first member. The seal comprises a support having a central axis and a plurality of plates attached to the support. Each plate has a first major surface and an opposite second major surface. The plurality of plates are arranged so that with respect to adjacent ones of the plurality of plates the first major surfaces confront the second major surfaces. In addition, the plurality of plates is attached to the support so that planes coincident with the major surfaces do not intersect the central axis.
Yet another aspect of the present invention is a brush seal intended for use with a first member. The seal comprises a support having a central axis and a plurality of annular plates attached to the support so that each of the plates is spaced a first axial distance from adjacent plates, as measured along the central axis, so as to form a cavity between adjacent plates. Each of the plates has an axial thickness, a radially inner end and a radially outer end, with at least one of the plurality of plates having a taper adjacent its radially inner end in which such axial thickness decreases from (i) a first amount proximate, but spaced from, the radially inner end to (ii) a second, lesser, amount at the radially inner end. The seal also includes a plurality of bristles positioned in at least some of the cavities.
Still another aspect of the present invention is a turbomachine that comprises a first region, a first member supported for rotation about a central axis, and a housing having a second region in which the first member is received. The second region is sized and configured so that the first member can rotate relative to the housing within the second region. The turbomachine also includes a seal of the type described above attached to the first member and the housing so as to maintain a pressure differential between the first region and the second region.
Other aspects of the present invention are described elsewhere herein and are illustrated in the accompanying drawings. As such, the preceding summary of the invention is illustrative and not limiting.